Clear Water and Filamentous Algae

The lake is really starting to clear up, and you can easily see the bottom in the East Arm by LOC offices. As of yesterday the Secchi reading for the Main Lake station was almost 11 feet, which means light is reaching the sediment throughout most of the near-shore areas. To put it in context, think of how much sediment is exposed during a typical LOC drawdown, and that is the amount of the lake bed that is seeing sunlight currently. Although water clarity is good and will likely get better, is around the middle of the road compared to readings from 2008 to current. The highest monthly average in the chart below was around 22 feet in May 2008, which is possible to reach if the lake continues to clear up this spring. We had well below average rain in March, which limits the amount of sediment in the water. Zooplankton are still actively grazing on diatoms, which helps clear the water even more. If we have a dry April we very well may see a decent increase in water clarity.

What else does the clear water do for water quality, other than high Secchi readings? It promotes the growth of filamentous algae, which is a benthic (at or near the sediment surface) species that grows in clear water ponds, lakes and streams. Although it starts growing on the sediment it can reach the surface if it becomes detached. This is typically caused by turbulence or very vigorous growth that causes the algae mats to float. You may notice this on very sunny days when oxygen bubbles become entertwined with algal filaments, causing strands to reach the surface.

There were a few days of warm weather last week that accelerated growth, and the pictures below were taken from the Lakefront shoreline across from LOC offices. The first image is algae growing in about four feet of water. The second image is dead and dying algae on the surface. When these mats reach the surface the sunlight bleaches them and causes rapid decay. Once the pelagic (growing within the water column) algae species start becoming active again the filamentous algae will be shaded out and stop growing as rapidly.